A widely used method for affixing toner materials to a receiver sheet is by the application of high temperature and pressure in the fusing subsystem of a photocopying machine. A common configuration for a fusing subsystem is to place a pair of cylindrical rollers in contact. The roller that contacts the side of the receiver sheet carrying the unfixed or unfused toner is known as the fuser roller. The other roller is known as the pressure roller. The area of contact is known as the nip.
A toner receiver sheet containing the unfixed or unfused toner is passed through the nip. A soft coating on one or both of the rollers allows the nip to increase in size relative to the nip which would have been formed between two hard rollers and allows the nip to conform to the receiver sheet, improving the fusing quality. Typically, one or both of the rollers are heated, either through application of heat from the interior of the roller or through external heating. A load is applied to one or both rollers in order to generate the higher pressures that are necessary for good fixing or fusing of the toner to the receiver sheet.
The application of high temperature and pressure as the receiver sheet passes through the nip causes the toner material to flow to some degree, increasing its contact area with the receiver sheet. If the cohesive strength of the toner and the adhesion of the toner to the receiver sheet is greater than the adhesion strength of the toner to the fuser roller, complete fusing occurs. However, in certain cases, the cohesive strength of the toner or the adhesion strength of the toner to the receiver is less than that of the toner to the fuser roller. When this occurs, some toner will remain on the roller surface after the receiver sheet has passed through the nip, giving rise to a phenomenon known as contamination. Contamination can also occur on the pressure roller.
In order to achieve desired image quality with respect to gloss, the surface properties of the roller are paramount. This is also true of an overcoated roller. The base cushion surface properties can affect the final, outer surface properties of the fuser member and therefore affect image quality.
There are two possible methods of making suitable fuser members. The first is to mold the fuser member or the fuser member base cushion. The advantage of this is that the surface properties of the fuser member or base cushion can be controlled by the quality of the mold surface. The disadvantage of this process is that a molded part will generally have problems associated with resin shrinkage and centering of the roller in the mold. The problems occur when the material is not perfectly centered on the roller. This results in paper handling problems as well as the possibility of an uneven nip. An uneven nip results in nonuniform fusing quality and gloss.
Multilayer rollers have been described to impart properties such as an oil barrier layer as described in U.S. patent application Ser. No. 09/056,027 and adhesion as in U.S. Pat. No. 5,534,347. Another role for an intermediate layer may be to separate two incompatible materials such as an addition cured from a condensation cured silicone as in U.S. patent application Ser. No. 09/056,027.
The second method for making a suitable fuser member is to coat an intermediate layer over a ground roller surface. This allows the tight control of the dimensional tolerance. In order to be able to produce a fuser member for a high quality image with desirable image characteristics such as gloss. One criterion is that the intermediate layer must be able to fill in all the pores of the ground surface. Another criterion is that the process should proceed quickly.
One difficulty in obtaining consistent high image quality with ground fuser roller material is that any roughness or variability in the grinding method and wheel, or the occurrence of a high frequency pattern from grinding, becomes apparent in the final image and this is undesirable. Intermediate layers have been mentioned as a method to control surface finish but they have not been described in detail.
There is a need for improved fuser members with improved fusing performance, e.g. increased coating quality to produce improved image characteristics without reducing the toner releasability, fuser member processability, temperature control, or dimensional tolerances.